In the electronics industry, electrical components such as resistors, capacitors, inductors, transistors, integrated circuits, chip carriers and the like, are typically mounted on circuit boards in one of two ways. In one way, the components are mounted on one side of the board and the leads from the components extend through the board and are soldered to the circuit on the opposite side of the board. In the other way, the components are mounted and soldered on the same side as the circuit, i.e. surface-mounted.
Solder paste is commonly used for surface-mounted soldering of electrical components to circuit boards. Solder paste is useful because it can be applied to selected areas of the circuit board and can be readily adapted to automation. Its tacky characteristic provides the capability of holding the electrical components in position without additional adhesives before forming the permanent bonds by soldering.
Solder paste typically comprises a solder powder, a resinous component such as rosin, activators such as organic acids or amines, rheological control agents, thickeners and solvents. The solder paste is typically coated on the circuit board by means of screen printing, dispensing, transfer printing and the like. Thereafter, the electrical components are placed on the circuit board and the solder paste is reflowed. The term "reflow", as used herein, means heating the solder sufficiently to cause it to melt and thereafter cooling the solder sufficiently to cause it to solidify.
One problem in the industry associated with the use of solder paste is that it often has a short and unpredictable shelf life, e.g., typically from about one month to six months. The unpredictability in shelf life is caused, at least in part, by variations in the lag time from when the solder powder is made to the time it is mixed with flux to form solder paste, thereby resulting in variations in the degree of oxidation on the solder powder. Such oxidized powder does not reflow as well as unoxidized powder and tends to agglomerate and form solder balls. Furthermore, when the solder powder is combined with flux, which is corrosive, the solder powder often reacts with the flux, thereby further oxidizing the powder and reducing the acidity of the flux. As a result, the solder paste often becomes less effective with time. Moreover, the reaction between the solder powder and the flux typically causes the viscosity of the solder paste to increase substantially, which can make printing of the solder paste difficult or impossible.
Attempts have been made to reduce the reaction rate between the solder powder and the flux and thereby increase the shelf life of the solder paste, by storing the solder paste under refrigeration conditions. However, refrigeration is not effective to compensate for the varying degrees of oxidation on the solder powder prior to its incorporation into the solder paste.
It has also been proposed to coat the solder powder with materials that are non-reactive with the solder paste. For example, U.S. Pat. No. 4,994,326, issued Feb. 19, 1991, discloses at column 4, lines 44 to 50:
The coating agents to be used are insoluble or hardly soluble in a vehicle for solder pastes to be described later, and may include those based on silicone and fluorine such as, for instance, silicone oils, silicone base high-molecular compounds, fluorinated silicone oils, fluorosilicone resins and fluorinated hydrocarbon base high-molecular compounds. PA1 When a carbon-containing compound is used as the coating agent, a quantitative analysis value for carbon is preferably 30 to 70 mol %.
The patent also discloses a relatively large mount of coating material which is applied to the solder powder. For instance, at column 5, lines 44 to 47, it is stated:
According to the above-referenced patent, the relatively large amount of coating material may be effective to inhibit oxidation of the solder powder. However, in general, large amounts of coating material are undesirable since they can create a barrier which can inhibit the reflow of the solder. Moreover, such large amounts of coating material may cause physical obstructions and/or impurities which result in poor reflow characteristics. In general, poor reflow characteristics are evidenced by the formation of solder balls and by the inadequate wetting of the substrate by the flux which can cause poor spreading of the solder and a discontinuous solder connection.
In addition, the above-referenced patent discloses the use of fluorinated hydrocarbons which are used as solvents in coating the solder powder. Currently, fluorinated hydrocarbons are considered to be an environmental pollutant and the use thereof is generally undesirable.
In view of the problems described above, improved solder pastes having coated solder powder are desired which can inhibit oxidation of the solder powder without substantially inhibiting the reflow characteristics of the solder.